1. Field of the Invention
The present invention relates to switchmode power supplies, and more particularly to resonant flyback converter switchmode power supplies.
2. Description of the Related Art
Switchmode power supplies for converting a first DC voltage to a second DC voltage are commonly used to power electronic systems. The first DC voltage may be from a DC voltage source, or from a rectified and filtered AC source. Such power supplies are generally used because of their relatively low cost, high efficiency, and high power density. The high power density is afforded by the high switching frequency used, enabling the use of much smaller, lighter, and lower cost magnetics and capacitors. The control circuits for such power supplies typically include a pulse generator or oscillator which generates a series of pulses whose width or frequency or both are modulated. These pulses are used to control the duration of applications of the DC input voltage across the power transformer in the normally single ended flyback power supply.
In general, a conventional switchmode power supply takes its input power from a DC voltage source which may be derived from an alternating current line via a rectifier filter combination. It consists of a power transformer including a primary winding and at least one secondary winding, a pulse generator for developing a train of pulses of varying pulse width as a function of the present value of the output voltage, and a transistor switch in series with the DC voltage source and the primary winding of the power transformer. This transistor switch is controlled by the pulses developed by the pulse generator such that the switch closes for the duration of each pulse. An attenuated sample of the output DC voltage is compared to a fixed reference voltage generating a voltage error, which is amplified by the error amplifier. This amplified error voltage either expands the width of the pulses in the pulse train to raise the output voltage, or narrows the width of the pulses to generate a lower DC output voltage, to thereby maintain the output DC voltage at the prescribed voltage level. Such feedback of the output DC voltage is necessary since otherwise the output voltage would vary inversely as a function of the varying demand of the load being powered by this output voltage and by variations in the input voltage. A rectifier and capacitive filter circuit are also connected to the secondary winding for smoothing the pulsating rectified output DC voltage.
One version of boost switching power supply topology is called a flyback power supply or flyback converter. In such power supplies, energy from the DC source is transferred first to the power transformer during a drive cycle and secondly to the output rectifier circuit during a flyback cycle, the output rectifier and filter circuit smoothing the output voltage. During the drive cycle, the transistor switch closes, thereby coupling a primary winding of the power transformer in series with the DC source. This develops a current in the transformer inductance causing energy to be stored in the transformer. No current flows in the secondary winding because of the reversed polarity the secondary winding applies to the rectifier circuit. When, however, the switch is opened, the current supplied from the DC source to the primary winding goes to zero and the magnetic field created by the current in the primary inductance starts to collapse. This induces a voltage of opposite polarity in the second winding, forward biasing the rectifier circuit, so it now conducts current that charges the output capacitor which powers the output load. The error amplifier compares the output voltage with a fixed reference voltage, causing the pulse width generator to develop the width modulated pulses to control the "on" time of the transistor switch during the drive cycle and thus the amount of magnetic energy stored in the power transformer.
The above-described concept of the resonant flyback using a single transistor is an old one, used in most TV sets. U.S. Pat. No. 4,616,300 to Santelmann discloses a circuit which supplies power to the load not only in conventional flyback mode, but also in the forward mode. This combination flyback/forward provides the alternate polarity energy pulses to the load required by high voltage multipliers used in CRT type high voltage converters.
In accordance with the Santelmann patent, the transistor switch is closed for a portion of each conversion cycle during which energy is delivered via the transformer to the voltage multiplier in the forward mode operation, and also energy is stored in the transformer magnetics to be delivered to the voltage multiplier during the flyback mode operation. The frequency and width of the modulating pulses produced by the control circuit of the Santelmann circuit are such that the natural frequency of the converter is tracked with the necessary duty factor. The natural frequency of the converter is the inverse of the natural period, which the sum of the subcycle intervals. These subcycle intervals are the charging of the magnetics and the forward mode energy delivery to the load; the resonant "ring" or flyback to the opposite polarity; the delivery of the opposite polarity energy to the load during flyback; and the resonant "ring" return to the starting condition. The resonant circuit controlling these "rings" is formed by the primary inductance of the transformer, and the stray capacitance of the secondary winding reflected to the primary of the transformer by the square of the secondary to primary turns ratio.
Referring to FIG. 3 of the Santelmann patent, the conversion cycle begins by turning on Q.sub.1, which places the input voltage E.sub.in across the primary inductance, L.sub.p, of the transformer and inductively across the secondary inductance, L.sub.s of the transformer (the turns ratio is assumed to be unity for simplification). During the time Q.sub.l is on, forward mode conduction is achieved in that the induced negative voltage across L.sub.s at the polarity dot is applied to the voltage multiplier load, causing current to flow in the voltage multiplier. This is unregulated in that the secondary voltage is related to the magnitude of the input voltage at that time.
Referring to FIG. 2 of Santelmann, which represents the flyback mode of operation, the items in FIG. 3 of Santelmann relative to FIG. 2 of Santelmann are Q.sub.1 is S, L.sub.p is L, C.sub.s reflected to the primary of the transformer is represented by C. A summary of the description of the Santelmann resonant flyback is that when switch S is closed, diode D is reversed biased and does not conduct, while current i linearly increases with time to its peak value, i.sub.pk, at which time the switch S is commanded open by the control circuit. The resonant circuit formed by C and L start their resonance as shown by Santelmann with the initial conditions of a voltage across C equal to E.sub.in, and a current through L of i.sub.pk. The resonance causes the voltage across the capacitor (which is equal to E.sub.in at Santelmann point e in the schematic) to decrease to zero, and the current in the inductor to increase slightly more to a value i.sub.pk2, representing the energy transfer from the capacitor to the inductor. The inductor current causes the locus of the voltage at node e to increase sinusoidally toward a peak voltage across the capacitor of opposite polarity from the initial voltage, and of a magnitude equal to the product of i.sub.pk2 and the characteristic impedance Z.sub.o of the resonant circuit formed by C and L, e.g. Z.sub.o =Sqrt (L/C). However, the design of the circuit for proper operation requires the output load voltage to be significantly lower than this peak. As shown by Santelmann's example, the 787 volt unloaded resonant peak across the capacitor is clamped at 318 volts across the capacitor by the load. In FIG. 3, while Q.sub.l is closed energy is stored in C.sub.s by the voltage impressed across it, and current linearly increases in L.sub.p from 0 volts to E.sub.in, at which the current in L.sub.p is at its peak, and then to reverse the voltage across the drain toward a voltage determined by this peak current and the characteristic impedance, as described previously. When the induced secondary voltage resulting from this resonance reaches an appropriate level, diodes D.sub.5, D.sub.4 and D.sub.3 conduct, recharging the multiplier capacitors. The magnitude of this recharge is controlled by the feedback circuitry providing the control to maintain the output voltage at the desired value.
When the inductor current reaches zero, there is no more current available for the forward conduction of D.sub.3 to continue and it opens. The energy that remains on the stray capacitor C.sub.s causes the voltage e to fall as a cosine waveform in free resonance. The voltage falls from the high peak value related to the output voltage, to a value equal to E.sub.in, during which time the current in L.sub.p increases, storing energy in the transformer core. This energy is then transferred to the capacitor as the voltage continues to resonate toward zero volts on the drain of the transistor. The magnitude of the energy stored in the inductor by design will normally cause the voltage to resonate to a negative value. However, it is limited to approximately zero volts by the clamping action of the body diode of the transistor. When the transistor body diode clamps, the remaining energy in the inductor is returned to the input voltage source. Completion of one cycle occurs when the current in L.sub.p has run down to zero. The start of the next cycle is the crossover of the L.sub.p current to a positive current, starting the storage of energy for the next cycle.
The capacitor C.sub.f (the combination of the multiplier circuit capacitors) is made very large, so that the voltage across the load R.sub.5 remains relatively constant throughout the cycle; in essence, C.sub.f acts as a battery, the feedback circuit serving to maintain a constant charge on C.sub.f by varying the duty cycle of the transistor Q.sub.l and thus the energy injected into the circuit in response to energy delivered to the load.
While the Santlemann circuit advantageously supplies power to the load in both the flyback and forward modes, it does so non-symmetrically; i.e., the output impedances, voltages and current differ for the two modes, because in one mode the transistor switch is conducting, while in the other mode it is not conducting.